The present invention relates to an apparatus for controlling paper transfer speed of a printing section of a form printing machine which carries out successive steps from multicolor printing on a paper web to processing, such as punching and perforating.
The whole structure of a multicolor form printing machine is shown in FIG. 13 of the appended drawings. In this Figure, there is illustrated a paper feeding section 1 for transferring a paper web P from a roll, a printing section 2 for achieving multicolor offset printing on the fed paper web P, a processing section 3 for processing, for example, punching, perforating and the like the printed paper web P which is transferred from the printing section 2, and a paper discharge section for discharging the printed and processed paper web into a zigzag folded stack.
In the printing section 2, printing units 5 are arranged on a printing line. The number of the printing units 5 coincides with the number of colors used in the multicolor printing, and in FIG. 13, four printing units which coincide with the four color printing are arranged. The paper web P is successively passed through each of the printing units 5 . . . and printed in multicolor. Each printing unit 5 comprises a plate cylinder 6, a blanket cylinder 7 formed of an elastic material such as rubber onto which images on the plate cylinder are transferred and a metal impression cylinder 8, these three cylinders being rotatably supported by a printing cylinder support in such a manner that their circumferential surfaces are substantially in contact with one another. The paper web P is passed between the blanket cylinder 7 and the impression cylinder 8, where the image transferred from the plate cylinder 6 onto the blanket cylinder 7 is printed on the paper web P.
As shown in FIG. 10, the rotatory power of a drive motor(not shown) is transmitted through a drive shaft 11 to each of the printing unit 5. The drive shaft 11 is extended over the paper feed section 1 and all of the printing units 5 . . . of the printing section 2. The rotatory power of the drive shaft 11 is transmitted through a transmission device 12 such as a worm gear mechanism to an infeed roll 10 and through a transmission gear 13 to the plate cylinders 6 of the printing units 5 . . . .
As shown in FIG. 11, spur gears 14, 15, 16 are mounted respectively on the rotatory shafts 6a, 7a, 8a respectively of the plate cylinder 6, the blanket cylinder 7 and the impression cylinder 8 of each printing units 5. And through the rotation of the spur gears 14, 15, 16 being in engagement with one another, the rotatory force of the drive shaft 11 is transmitted to the plate cylinder 6, the blanket cylinder 7 and the impression cylinder 8. Thus, these cylinders 6, 7, 8 are rotated.
In this case, the speed of transfer of the paper web P(hereinafter referred to as the paper transfer speed) of the printing unit 5 is controlled by the rotation speed of the blanket cylinder 7 and the impression cylinder 8, but it is also changed in accordance with the change of the thickness of the paper web P. Namely, in FIG. 12, if d is the diameter of the impression cylinder 8; N is the rotation frequency of the impression cylinder 8; t is the thickness of the paper web P; and V is the paper transfer speed, a formula EQU V=N(d+t).pi.
is obtained. And therefore, as the thickness t of the paper web P (hereinafter referred to as the paper thickness t) increases, the paper transfer speed V rises and the paper transfer amount per a unit time increases.
In the case of the blanket cylinder 7, the change of the paper thickness is compensated by the diameter change through the elasticity of the blanket cylinder 7, and therefore, the paper transfer speed is changed by the relation of the nonelastic impression cylinder 8 and the paper thickness. In other words, the paper transfer speed is determined by the rotation speed of the impression cylinder 8 and the paper thickness. Further, generally in a business form printing machine, the paper thickness changes within the range from 0.05 mm to 0.2 mm. Therefore, relating to the paper transfer speed V, the following formulas are obtained.
Minimum value EQU V min=N(d+0.05).pi.
Minimum value EQU V max=N(d+0.2).pi.
In a conventional printing machine, however, since the impression cylinder 8 is always rotated together with the blanket cylinder 7 at a predetermined speed, independently of the paper thickness, the paper transfer speed changes with the change of the paper thickness. As a result, the following problems have been caused.
1. In the printing unit 5, monocolor or multicolor, the change of the paper transfer speed with respect to the blanket cylinder 7 rotating at a constant speed (i.e. the change of the printing pitch) causes the unevenness of the printing precision.
2. Due to the change of the printing pitch, the tension of the paper web P of the printing section 2 become different from that of the foregoing paper feed section 1 or of the following processing section 3. Consequently, a high tension is applied to the paper web P and a paper transfer mechanism, which causes the paper web P to be broken and the life span of the paper transfer mechanism to be shortened.
3. In multicolor printing, the difference of the diameters of the impression cylinders of the printing units 5 causes the difference of the paper transfer speed between the printing units i.e. the difference of the printing pitch therebetween, whereby the similar problems have arosen.
An object of the present invention is to overcome the abovementioned problems by providing an apparatus for controlling paper transfer speed of a printing section of a form printing machine.
Other objects and advantages of the present invention will become apparent from the following description.